Remove weird properties on the WebVTT cue background box

These were added in commits 3d05e2e05c72b01404cf290bf07c7f0f367827a7 and
be09934775d380753873ab9667c70e1b3af43aa3 but don't make sense since the
WebVTT cue background box is a per-cue inline box.
Lowercase the "apply WebVTT cue settings" definition

For consistency with e.g. "collect WebVTT cue timings and settings"
Refactor how and when "apply WebVTT cue settings" is invoked

This factors out the common "Let nodes be" and most importantly makes
the algorithm actually return the CSS boxes.
Remove hidden comments for cue and region settings

These aren't particularly illuminating, and the indentation would get
mangled by format.py. Fixing format.py does not seem worthwhile.
Move the ruby anonymous box logic back to its original place

This was moved in commit 3d05e2e05c72b01404cf290bf07c7f0f367827a7
but seems to belong better in its original place since it creates
boxes and does not merely set properties.
Remove the region nullification from the parser

Scripts can already get cues into the state that this tries to avoid,
and it seems harmless due to how the rendering algorithm works.

https://www.w3.org/Bugs/Public/show_bug.cgi?id=25948
Merge pull request #83 from foolip/rm-setting-comments

Remove hidden comments for cue and region settings
Add missing space in 'to height'
Separate the steps for cue and region lists in rendering

This was split in a slightly odd way before...
Merge pull request #89 from foolip/to-height

Add missing space in 'to height'
Merge pull request #90 from foolip/let-regions

Separate the steps for cue and region lists in rendering
Drop 'cue' from 'A cue line position/alignment'

No other terms in this section except 'A cue box' include 'cue'.
Quote strings in the syntax section for consistency

Compare with e.g. the "align" setting.
Merge pull request #93 from foolip/quote-strings

Quote strings in the syntax section for consistency
Merge pull request #91 from foolip/a-cue-line

Drop 'cue' from 'A cue line position/alignment'
Merge pull request #82 from foolip/reorganize-rendering

Refactor how and when "apply WebVTT cue settings" is invoked
Introduce a positionAlign value of "auto"

"auto" is the default positionAlign setting
It depends on the "align" setting.
Introduce "computed text position align"

allows removing of positionSetAlign in the rendering section
Merge commit '174d9919bafc482d15b22c224c9d82dc8eb631d0' into revert-position-size-align
Merge commit 'caa71756bec73aabcef2177c3ea622dd68c07f5d' into revert-position-size-align

Conflicts:
	webvtt.html